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Re: [Amps] FW: High filament current on the 8877 amplifier tube

Subject: Re: [Amps] FW: High filament current on the 8877 amplifier tube
From: MU 4CX250B <>
Date: Mon, 9 Oct 2023 22:36:50 +0000
List-post: <>
Matt, I believe the W8JI precautions you cited are only half correct.
Yes, it is important never to draw cathode current from a tube with an
underheated filament or cathode. However, it is fine to apply plate
voltage to a tube with a cold filament, so long as there is no cathode
current. I’m fact, many if not most commercial amplifiers (e.g., the
Alpha 9500), turn on their filament and high voltage supplies
simultaneously, and I have done this with my homebrew amplifiers for
decades. Of course, one should also apply cutoff bias to the tube
while it is warming up, and for tetrodes it is a good idea to
interlock the screen voltage supply so that it can’t turn on unless
the plate voltage is on. The important point is not draw cathode
current until the tube warms up completely.
Jim w8zr

Sent from my iPhone

> On Oct 9, 2023, at 2:20 PM, wrote:
> I have nothing to offer about the excessive current draw, but I have read a 
> lot of warnings NOT to apply plate voltage to an 8877 until the cathode comes 
> up to full temperature to avoid damaging the oxide cathode.   "Do not apply 
> HV or draw cathode current unless the heater has been within factory 
> specified range for 3 full minutes.   If the heater is started very slowly, 
> the tube warm-up time should be extended.   Never run a (8877) tube when the 
> heater voltage is low.   Doing so will poison the cathode"  -
> Matt
> -----Original Message-----
> From: Amps <> On Behalf Of MU 4CX250B
> Sent: Sunday, October 8, 2023 4:12 PM
> To: Tim Duffy <>
> Cc:;
> Subject: Re: [Amps] High filament current on the 8877 amplifier tube
> Hi Tim,
> I’ve never heard of that happening, though I’ve not investigated the 
> possibility with my own 8877s. I’m wondering if the date codes and 
> manufacturer for the odd tubes are different from your known good tubes? 
> Speaking from experience, a  small dent in the anode cooler shouldn’t hurt 
> anything, at least for EIMAC tubes, but one never knows for sure.
> If you can do it easily, you could try applying plate voltage with the
> 8877 filament turned off, to test for a cold short in the tube. If ok, then, 
> leaving the HV on,turn on the filament and cutoff bias to see what happens as 
> the tube warms up. You should see no cathode current or grid current. If the 
> tube passes this test, but hasn’t been used for a couple of years, or if it’s 
> a chinese or soviet tube, then just let it cook overnight before applying 
> drive power. Make sure you’ve got a fuse in the plate lead to the tube, or 
> else be sure to have a fire extinguisher handy!
> 73,
> Jim w8zr
> Sent from my iPhone
>> On Oct 8, 2023, at 12:48 PM, Tim Duffy <> wrote:
>> Most 8877 tubes that I test draw 11 amps of filament current at 5 volts.
>> I have tested three recently that draw 15 amps of filament current.
>> All of these tubes are used and have little "dings".
>> Is 15 amps an indication of a problem tube? Or should I apply HV to
>> the plate and see what happens?
>> 73
>> Tim K3LR
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